so these items aren't actually necessary they just act as a slap in the face to those who dare try be a tank
Zed was garbage tier and AD Casters as a whole had serious viability issues before Cleaver was changed to a %armor shredding CDR item.
Regardless of how crumby it feels, %armor penetration being prevalent enough to give AD casters a strong tertiary multiplier that can keep up with Crit on marksman and the party-hat/%magic pen/Lich Bane multipliers on the AP side is necessary.
Also: Just as you can say %armor pen devalues armor: so to does AD, Crit, AS.
And HP, armor, %AS slows, %Crit reductions, %AA reductions all devalue those offensive stats.
The interaction with %armor pen and defense isn't particularly unique in regards to any other offensive/defensive arms race. And you need 50+50% armor penetration to double damage against anyone with 100+ armor, where you need ~150% AS or 67% crit with IE to double DPS with AAs.